The invention will be elucidated below for brazing sheet scrap, but the method can be used as well for other types of metallic coating layers on a metallic core.
During the production of brazing sheet a plate of an aluminium alloy having a relatively low Si content for the metallic core of the brazing sheet is on one or both sides clad by means of roll bonding with a plate of an aluminium alloy having a high Si content for the clad layer on the metallic core. This sandwich of metallic core plate and clad plate(s) is subsequently rolled so as to bind the clad layer(s) to the metallic core layer and to produce the brazing sheet product having a thickness of typically between 0.1 and 3 mm, for use in the production of for instance heat exchangers for automobiles.
During the production of the brazing sheet significant amounts of scrap are produced, for instance the heads and tails of the sandwich plates after each hot or cold rolling operation. Because the scrap contains both aluminium alloys with a high Si content and aluminium alloys with a low Si content, simple melting of the scrap would result in an aluminium alloy having a raised Si content as compared to the Si content of the metallic core, which is too high to be used for producing similar type metallic core plates, unless diluted with substantial amounts of alloys having a very low Si content.
Another source of brazing sheet scrap is formed by used products that are made out of brazing sheet, such as used heat exchangers.
Various methods are available to de-coat the clad alloy from the metallic core alloy in the scrap. One of these methods is described in international application no. WO 99/32260. According to this method, the metallic coating layer is separated from the metallic core, by rotationally tumbling or shaking the scrap metal pieces in a container together with abrading particles such as to agitate the scrap metal pieces together with the abrading particles and thereby causing multiple collisions, whereby the metallic coating layer is at least partially removed from the metallic core. During the agitating, the container is held at a temperature whereby the temperature of the scrap metal pieces is higher than the solidus temperature of the metallic coating layer and lower than the liquidus temperature of the metallic core.
It is a disadvantage of the known method that thin gauge material, in particular sheet material with a thickness gauge of less than 2 mm, is difficult to process due to excessive wear on the thin gauge by the abrasive particles, resulting in a complete loss of material.
It is another drawback of the known method, that one or more alloy elements diffuse during processing at elevated temperature from the metallic coating layer to the metallic core, thereby further contaminating the metallic core material.
It is another disadvantage of the known method, that there is a risk that removed metallic coating material adheres to the abrading particles. This results in a reduction of abrasive properties.